The Mystery of Handedness, Solved!: A Brain-based Explanation of Left-Handedness (and Language Lateralization)
A Quick Overview
The “novelty-routinization” theory of brain lateralization posits that the neocortex’s left and right hemispheres primarily process cognitively routine and cognitively novel tasks, respectively. (Goldberg 2018)
Consequently, the left hemisphere excels at fine motor control, as it deals primarily with “cognitively routine” functions. And since the left hemisphere controls the right hand, most of us come to be right-handed due to the left hemisphere’s finer control of the movements of its associated hand, the right hand.
In contrast, left-handers did not develop the left hemisphere sufficiently enough prior to establishing a preferred hand. As such, the left hemisphere had yet to develop finer control of the right-hand. This leaves such people to prefer their genetically preferred left-hand instead. (Thus, in general, only those with a genetic factor for left-sidedness can become left-handed; if a person has the genetic factor for right-sidedness, the person becomes right-handed regardless.)
Note/Support: As we detail in a related article, similar occurs regarding language lateralization. In short, most of us learn enough language early on for it to come to reside in the left hemisphere (i.e., language, in general, becomes “cognitively routine” to us, and thus it comes to be in the left hemisphere).
Akin to left-handers, though, a small percentage of individuals fail to acquire enough language proficiency during their early years. For these individuals, language processing takes root in the right hemisphere. In other words, due to a slower learning rate of language, the typical shift of language information from right-to-left fails to occur (at least to some degree), resulting in language residing instead more so in the right hemisphere.
The Science of Handedness: Exploring Genetic Predisposition and Brain Development
In line with the handedness patterns seen in most other mammals, human handedness would likely be evenly distributed (close to a 50–50 split) if genetics were the sole factor at play. However, in contrast to this genetic expectation, approximately 90% of the population tends to be right-handed, leaving about 10% as left-handed individuals. This discrepancy suggests that most of us (roughly 80%; 40 out of 50) with the genetic predisposition for left-handedness counter this trait.
Based on this theory of laterality, the genetic factor for left-handedness is countered if the neocortex’s left hemisphere (left-brain) is developed sufficiently enough prior to the establishment of a preferred hand. The left-brain’s development can counter this gene due to the left-brain being the routinization side (Goldberg 2018). As such, in principle it has superior natural ability for finer discrimination and control of movements.
Thus, if genetic left-handers sufficiently develop the left-brain in early childhood, it leads them to have greater control of its corresponding hand, the right hand. This leads it to become their preferred hand, thereby countering their genetic inclination for left-handedness.
In comparison, those of us that are left-handed did not develop the left-brain sufficiently enough prior to the establishment of a preferred hand. In these cases, the genetic predisposition for left-handedness was not countered.
In essence, the rate of learning during early childhood plays a pivotal role in determining the level of left-brain development. If this learning rate is fast enough, enough is learned such that the left-brain develops sufficiently enough to counter a genetic factor for left-handedness in people that have it. Otherwise, such people become left-handed.
Given the inviolate right-to-left shift of information in the brain (Goldberg 2018), one’s learning rate in early childhood also dictates the development of the right-brain. The right-brain, though, predominantly handles novelty (Goldberg 2018), so its development does not lead to greater control of the left-hand compared to the right-hand of a genetic right-hander. Thus, genetic right-handers become right-handed regardless, some neurologically atypical cases aside (as discussed in another article).
General note: As detailed in another article, the underlying cause of a slower learning rate might be brain-related differences. Such differences would explain handedness-related findings in fetal and neonatal subjects.
Moreover, although these brain-related differences might cause a slower learning rate in early childhood, they perhaps lead to advantages later in life. This perhaps helps explain any relatively greater intelligence or creativity in some left-handers. And it also fits with the relatively higher percentage of left-handers found in the science/engineering community as well as in the arts.
Support from Captive Chimpanzees
Similar to humans, chimpanzees in captivity have shown a preference for the right hand. With captivity environments such as research centers providing an improved learning environment, these chimpanzees have the opportunity for more extensive brain development, particularly in the left hemisphere.
This enhanced neural development often occurs before these chimpanzees establish a preferred hand. Consequently, a greater proportion of them tend to become right-handed, as some of the ones with the genetic factor for left-handedness counter it, just like in humans.
Support from Animals Lacking a Corpus Callosum
The corpus callosum serves as the primary bridge connecting the left- and right-brain, facilitating the transfer of information from right to left as it becomes cognitively routine. When a mammal lacks this structure (whether due to inherent traits, agenesis, or surgical transection), a propensity for left-handedness tends to emerge. Notable examples include wild kangaroos, other macropod marsupials, and mice afflicted by callosal defects, agenesis, or surgical transection.
This fits with this theory of laterality but is the mirror image of what occurs in regard to most genetic left-handed humans becoming right-handed: In particular, due to a lacking corpus callosum (and thus the lacking right-to-left shift of information in the brain), the right-brain develops in a manner that, upon the establishment of a preferred hand/paw, most of the animals become left-handed/pawed. This includes the animals with a genetic factor for right-handedness/pawedness, as the right-brain’s development counters that factor.
Note: A related presumption is that the left-brain in such animals does not sufficiently develop prior to the establishment of a preferred hand/paw. This fits with the presumably lesser right-to-left shift in the brain of information that occurs during learning in these animals, due to their reduced or absent corpus callosum.
Support from Cross-dominance
In so-called cross-dominance, the development of the left-brain can be enough to counter a genetic factor for left-side laterality, but only partially.
Take mixed-handedness, for instance. This occurs when genetic left-handers develop the left-brain enough in early childhood to come to prefer the right hand for some tasks but not for others. Such people have a related learning rate during early childhood that is not fast enough to completely counter their genetic factor for left-handedness, yet not slow enough to result in it dictating left-handedness in all regards.
Similarly, consider right-footed left-handers. Such people did not develop the left-brain enough to counter their genetic factor for left-handedness, but they did develop it enough by the time they established a preferred foot. This is because the learning period prior to the establishment of a preferred foot is likely longer than the one for a preferred hand.
In contrast, the learning period prior to the establishment of a preferred eye and ear is likely shorter than the one for a preferred hand. This explains why fewer of us with the genetic factor for left-side laterality counter it in regard to these features. In essence, people tend to establish a preferred ear and eye before the left-brain has the chance to develop sufficiently to counteract the genetic predisposition for left-side laterality in these respects.
(It’s essential to note, however, that in all of these cases, other atypical factors can also contribute to an individual’s preferences and, at times, even dictate them.)
Support from Various Groups of Neurologically Atypical People
Many neurologically atypical people likely have a slower learning rate in early childhood. This leads relatively fewer of them to develop the left-brain sufficiently enough to counter a genetic factor for left-handedness (if they have it). Examples include the higher incident of left-handedness in the mentally challenged as well as people with conditions such as epilepsy, Down syndrome, autism, and dyslexia.
Support from Gender-based Disparities in Handedness
Compared to women, men tend to more often be left-handed. Based on this theory then, girls have a faster learning rate in early childhood, at least in regard to handedness-related fine motor skills. Support for this comes from studies showing that girls develop sooner than, and are superior to, boys with respect to fine motor skills.
Perhaps boys have a slower learning rate in part due to males having a relatively lesser corpus callosum. This perhaps results in a relatively slower right-to-left shift of information in the brain during learning, which results in a relatively slower learning rate in early childhood. In addition, the male brain being less symmetrical / more asymmetric is perhaps a contributing factor or even the dictating one.
Another possible contributing factor (and perhaps even the dictating one) might be related to the quality of one’s senses. Females tend to have relatively sharper senses than males. This perhaps leads them to have a relatively faster learning rate in early childhood, given such learning tends to be primarily sensory-based (e.g., When I bang on the table, it makes a loud noise; I don’t like the taste of that green mushy food; After I push that button, I hear music; etc.).
Support from Identical Twins Discordant for Handedness
Based on this theory, in cases of identical twins discordant for handedness, only one of the twins develops the left-brain enough to counter the genetic factor for left-handedness (both twins, though, have this genetic factor). This difference in development could be due to numerous factors, perhaps most notably one baby having received comparatively more hormones in utero.
Note: If identical twins that are neurologically typical have a genetic factor for right-side laterality, they will not become discordant for handedness, as they both will be right-handed regardless of brain development rates (uncommon cases, aside).
Support from Non-heterosexuals
Findings indicate that non-heterosexuals might be exposed to more hormones in utero. This could lead to a slower learning rate in early childhood, such that those with a genetic factor for left-handedness do not counter it. Consequently, this phenomenon helps explain the increased occurrence of left-handedness among non-heterosexual individuals.
Note: Similarly, some neurologically atypical groups, such as autistic people, might be exposed to more hormones in utero too, and they too tend to more likely be left-handed.
Partial Support Related to One’s Learning Rate in Early Childhood
Based on the following study, the right-brain is more active initially in early childhood, but after age three, the left-brain is more active. Given a preferred hand becomes established around ages 4–5, this fits with this theory — by age four, most children have developed the left-brain enough such that those with a genetic factor for left-handedness counter it.
(Chiron et al. 1997)
“Between 1 and 3 years of age, the blood flow shows a right hemispheric predominance, mainly due to the activity in the posterior associative area. Asymmetry shifts to the left after 3 years.”
In addition, based on the following, it seems right-handed children in early childhood might have advanced patterns of cognitive development compared to left- and mixed-handed ones. This fits with this theory, in that the right-handers had a relatively faster learning rate in early childhood compared to left- and mixed-handers.
(Michel et al. 2016)
“We present evidence that children with consistent early hand preferences exhibit advanced patterns of cognitive development as compared to children who develop a hand preference later. Differences in the developmental trajectory of hand preference are predictive of developmental differences in language, object management skills, and tool-use skills. As predicted by Casasanto’s body-specificity hypothesis, infants with different hand preferences proceed along different developmental pathways of cognitive functioning.”
Similarly, based on the following, it seems left- and mixed-handed children perform significantly worse in nearly all measures of development than right-handed children. This fits with this theory, as the former have a relatively slower learning rate in early childhood compared to the latter. (And its finding of the disadvantage being larger for boys than girls fits with the earlier explanation for why girls are less likely than boys to be left-handed.)
(Johnston, Nicholls, Shah, & Shields 2009)
“We also find robust evidence that left-handed (and mixed-handed) children perform significantly worse in nearly all measures of development than right-handed children, with the relative disadvantage being larger for boys than girls. Importantly, these differentials cannot be explained by different socioeconomic characteristics of the household, parental attitudes, or investments in learning resources.”
Previously Hidden Issues Related to Studies Involving Handedness
Traditionally, handedness studies have focused on the dichotomy between right-handed (approximately 90% of the population) and left-handed (around 10% of the population) individuals. However, when we delve deeper into the numbers, intriguing patterns emerge.
Consider that, according to the presumed 50–50 split in the population, roughly 44% of the right-handers (40 out of 90, based on a total population of 100) are genetic left-handers. Similarly, based on 10% of the population being left-handed, 20% of genetic left-handers (10 out of 50, based on a population of 50 genetic left-handers) physically demonstrate their related gene.
Now, here’s where it gets interesting. Since 20% of genetic left-handers fail to develop sufficiently enough to counter their genetic factor for left-handedness, it seems plausible that 20% of genetic right-handers would experience similar brain development (associated with a lower learning rate). Thus, presumably 11% (10 out of 90) of the right-handed population would have brain development similar to left-handers. (Note: One’s genetically favored side of the brain, though, might be yet another factor in need of consideration.)
If so, identifying this 11% subset of the right-handed population would presumably result in far more consistent data in studies related to differences between two groups of people (no longer, though, would this be a comparison of right- and left-handers; instead, the two groups would presumably be those with a slower learning rate in early childhood compared to those with a faster learning rate, at least in regard to fine motor skills).
Note: As described in a separate article, atypical language lateralization might be yet another way to separate people presumably based on their learning rate in early childhood, albeit specifically in regard to language.
Closing Remarks
In addition to the related, further-supporting article on how this theory also applies to language lateralization, I invite you to explore my other articles that reinforce/support the “novelty-routinization” theory of lateralization.
In particular, one article provides a much-needed neurologically-based deeper understanding and associated revision to the thinking that we’re left- or right-brained (Spoiler: We are!). Another article provides fresh insights and brain-based explanations for sex-related differences, while a third one led to a brain-based unifying model of personality types / societal roles!
Feel free to ask any questions or contribute to the discussion otherwise. Together, let’s continue unraveling the mysteries of the human brain and uncover the profound implications they have for our lives.
Works Cited:
